Modern airborne radar, electronic counter measures and electronic warfare applications frequently call for the ability to transmit/receive a beam of electromagnetic energy within approximately 20 degrees (near end-fire) of the longitudinal axis of the airborne craft carrying the radar, ECM and EW systems. It is also frequently desirable to scan a beam of electromagnetic energy in one direction relative to the antenna's boresight axis, either simultaneously or switchably at electronic speeds. Further, in many applications, it is useful to have an antenna system that can operate over a relatively wide bandwidth, obviating the need for more than one antenna system.
In the past, linear array antennas whose boresight axis was perpendicular to the longitudinal axis of the airborne craft were usable for scanning a beam of electromagnetic energy to end-fire (90 degrees from the antenna boresight axis) because of the excessive loss in antenna gain due to the radiation characteristics of the elements of the array at the scan angles of interest. Hence, attempts to meet such operational requirements have been made by a variety of antenna systems. One such antenna system is a hog-horn antenna, which combines a horn antenna with a section of a parabolic reflector. A hog-horn antenna is typically shrouded in a blade-shaped radome which is mounted on the exterior of the fuselage of the airborne craft. In this configuration, the hog-horn antenna both adds aerodynamic drag to the airborne craft and contributes significantly to the reflective radar cross section of the airborne craft.
In another antenna, an array of eight or more vertically polarized radiators can be used to scan an antenna beam electronically between 20 and 30 degrees from the longitudinal axis of the airborne craft. This array produces a desirable cosecant radiation pattern in the elevation angle, controlling the fall-off rate of the elevation radiation pattern near the airborne craft's longitudinal axis. However, a vertically stacked waveguide array does not provide broad bandwidth coverage and suffers from high voltage standing wave ratio (VSWR) characteristics, which contribute to inefficient transmission of electromagnetic energy. Further, such a stacked waveguide array is mounted in a blade-shaped radome and is also characterized by high aerodynamic drag and an undesirably large radar cross section.
In another approach, cavity-mounted sectoral E-plane horns have been developed to increase the operational bandwidth while reducing tile radar cross section and VSWR. Such horns are typically very long relative to their aperture, which is covered by a dielectric or metal lens. This is necessary to control the rate at which the phase of the electromagnetic energy tapers across the horn aperture. The phase taper, in turn, controls the side lobes of the antenna radiation pattern. Therefore, the use of sectoral E-plane horns is hampered by their physical size requirements.
Accordingly, it is desirable to have an antenna system which can be made conformal to the fuselage of the airborne craft while producing an electromagnetic energy beam which can be directed to within 30 degrees of the airborne craft's longitudinal axis.